A number of devices are available for delivering a drug into the lungs of a patient. Once such device is a nebulizer, which is a device that is used for converting a liquid, such as a liquid medication, into an aerosol which is then inhaled by the patient, typically through a mouthpiece. A number of different types of nebulizers exist, such as, without limitation, jet nebulizers and ultrasonic nebulizers. A typical jet nebulizer uses compressed air to generate the aerosol from the liquid. One type of ultrasonic nebulizer employs acoustic waves having an ultrasonic frequency that are directed to a point on the surface of the liquid that is to be converted into an aerosol. At the point on the surface of the liquid where these ultrasonic waves converge, they will produce capillary waves that oscillate at the frequency of the ultrasonic waves. If the amplitude of the waves is large enough, the peaks of the capillary waves will break away from the liquid and be ejected from the surface of the liquid in the form of droplets, thereby forming the aerosol. A device that is often used for generating ultrasonic waves in an ultrasonic nebulizer is a piezoelectric transducer (such as a piezoelectric crystal), which vibrates and generates ultrasonic waves in response to an applied electric field. In another type of ultrasonic nebulizer, the liquid that is to be converted into an aerosol is forced through a mesh (thereby creating liquid droplets) by the vibration of a piezoelectric crystal acting upon a horn. In this type of ultrasonic nebulizer, the gauge of the mesh determines the size of the droplets which are created to form the aerosol.
Conventional nebulizer systems provide a continuous aerosol/drug output, and thus the amount of drug inhaled is dependent upon the patient's breathing pattern. The duty cycle of the patient's breathing pattern is typically 40:60. This means that the patient spends 40 percent of a single respiratory cycle in inspiration and 60 percent of the time in expiration. Thus, 60 percent of the drug delivered from the nebulizer will be wasted to the environment during expiration. In addition, the breathing pattern of a single patient over the course of a treatment will vary. In order to address these issues, more sophisticated nebulizer systems have been developed which adapt the delivery of aerosol to the patient's breathing pattern, delivering medication only when the patient is inhaling through the mouthpiece.
Adaptive nebulizer systems as just described have been developed which are capable of a number of different modes of operation. For example, one such system is capable of operating in either a tidal breathing mode and a target inhalation mode.
In the tidal breathing mode (TBM), the nebulizer system monitors the flow and inhalation time for the first few breaths (.e.g., three breaths) of each treatment. This information is used to predict how long the next breath is going to be. Once this has been calculated, aerosol is emitted into the beginning of the next inhalation. The prediction is updated after each new breath to ensure accuracy through the whole of the treatment. In a typical implementation, the device will emit aerosol into approximately 50 to 80 percent of each inhalation. In this mode, very little of the medication is wasted to atmosphere because the aerosol is emitted only when the patient is breathing in.
In the target inhalation mode (TIM), the nebulizer system encourages each patient to inhale for as long as they can, as this can result in a greater amount of the medication getting into the lungs, and can also reduce the treatment time. In particular, the patient is instructed to breathe in through the mouthpiece until a signal, such as vibration through the mouthpiece, is provided. The time between the start of the breath and the signal is called the target inhalation time—in other words, how long the patient should inhale. At the beginning of the first treatment, the target inhalation time is set to predetermined time, such as three seconds. If the patient is able to inhale past the target inhalation time, then the target inhalation time for the next breath is made a little longer. In this way, the duration of the breath is gradually increased until the patient reaches a target inhalation time that is suited to his/her own capabilities. If the patient is not able to inhale past the target inhalation time, then the target inhalation time for the next breath is made a little shorter. Also, there is always a gap, such as a two second gap, between the end of aerosol production and the target inhalation time signal to ensure that substantially all of the aerosol reaches the patient's lungs. One particular implementation of a nebulizer system which is able to operate in a target inhalation mode is described in United States Patent Application Publication No. 2006/0243277, entitled “Inhalation Method and Apparatus” and assigned to the assignee hereof, the disclosure of which is incorporated herein by reference.
Furthermore, the target inhalation mode is typically operated at a fixed inhalation flow rate, e.g., 15 l/min, which is lower than the inhalation flow rate of the tidal breathing mode, which can be as high as 80 l/min. It has been discovered that this difference in flow rates, particularly in the locality where the aerosol plume is generated, results in the aerosol particle size in the target inhalation mode being larger than the aerosol particle size in the tidal breathing mode. As will be appreciated by those of skill in the art, the smaller the particle size of the aerosol, the greater the lung deposition of the medication, as less medication will get trapped in the patient's upper airway and more medication will reach the periphery of the patient's lungs. Thus, it would be advantageous to be able to reduce the particle size of the aerosol that is generated by a nebulizer system that is operating at a given, fixed inhalation flow rate, such as a nebulizer system that is operating in the target inhalation mode at a 15 l/min inhalation flow rate, and, as a result, enhance lung deposition.